Worldwide demand for energy continues to increase. The search for petroleum is reaching greater extremes for well depths, pressures, and temperatures. High-pressure, high-temperature (HPHT) drilling techniques are often used today. HPHT wells are often classified into three temperature categories: Tier I, up to 350° F. (177° C.); Tier II, up to 400° F. (204° C.); and Tier III, up to 500° F. (260° C.). Most HPHT wells also have high reservoir pressures, e.g., up to 20,000 psi (1379 bar) for Tier II wells. The elevated temperatures and pressures in these wells place demands on equipment selection, downhole pressure determination, lost circulation treatments, and the like.
Drilling fluids, often referred to as drilling muds in the petroleum industry, are often used in well-drilling operations. The drilling fluid, which may be a water-, oil-, or synthetic-based formulation, circulates within the well bore, carries cuttings to the surface, lubricates the drilling equipment, and acts as a cooling agent. So-called “lost circulation” occurs when drilling fluids or muds enter pores or fractures in a formation, and are then lost to the drilling operation instead of returning to the surface for recycling and reuse. Lost circulation is a significant industry problem. It is estimated that lost circulation costs the industry about $1 billion per year in the United States alone, and that lost circulation products could represent as much as an additional $250 million annually. (Comparable figures for lost circulation costs are not immediately available for other countries, but such costs are believed to be substantial worldwide.) Lost circulation can lead to failures in testing wells, and can also lead to decreased productivity.
A variety of “lost circulation materials” (LCM) have been employed to try to reduce levels of lost circulation. These “lost circulation materials” have included such things as coarse cellulosic fibers, fine cellulosic fibers, coarse nut shells, synthetic graphite, cellulose derivatives, mineral fibers, fine calcium carbonate (e.g., 1 mm in diameter), medium-sized calcium carbonate (e.g., 2 mm in diameter), and coarse calcium carbonate (e.g., 3 mm in diameter). Lost circulation materials often include different particle types and sizes to address different sizes of fracture. If part of the material is rigid but compressible or expandable under pressure, the material can perhaps mold itself into fractures to help seal leaks. Cellulosic fibers alone will often lack the needed rigidity, especially in HPHT wells. To compensate for the lower rigidity, a greater concentration of the material may be required to inhibit lost circulation effectively. Due to the low density of cellulose, cellulosic fibers are often pelletized when used as lost circulation materials to reduce transportation, storage, and handling costs. Cellulosic materials can also be vulnerable to degradation by decay, mold, and insects (e.g., termites) during storage. Inelastic materials such as waste plastics and rubber used in lost circulation materials typically do not have the swelling/expansion properties that are needed to adequately plug cracks and fissures. Resilient graphitic carbon (RGC) of various sizes has been used in lost circulation compositions for its resilience, but RGC can be expensive.
LCMs are often classified according to their physical nature and mechanism of action as being granular, lamellar, fibrous, or encapsulated. A granular LCM forms bridges both at the formation face, and within the formation matrix. The effectiveness of granular LCMs depends on their particle size distribution (PSD). Larger particles can first form a bridge across or within a void, and smaller particles can then bridge openings between the larger particles. A blend of large, medium, and small particles is therefore often used. Fibrous materials are best used for controlling losses in porous and highly permeable formations by forming a mat-like bridge over the pore openings. The mat reduces the size of the openings in the formation, permitting colloidal particles in the mud to rapidly deposit a filter cake. Flake (or lamellar) LCMs bridge voids and form a mat on the formation face, and are typically used for permeable and porous formations. A reactive LCM can be encapsulated within a chemical barrier to allow the material to be pumped through the drill pipe without reacting with the drill pipe itself. For example, lime can be encapsulated by an organic wax that has a melting point below the temperature of the underground formation, but above the maximum temperature of the drilling fluid that circulates within the borehole. The lime is not released and remains essentially unreactive while in the borehole. Once the lime reaches the underground formation, the wax melts and the lime becomes accessible for reaction. Blends of granular, flake, and fibrous LCMs have also been used, to supply varying particles size and material types for sealing different types of lost circulation zones. Combining different materials, however, usually tends to increase material handling costs.
How well a plug prevents fluid loss in a fracture depends upon both the mechanical strength of the plug and its permeability. The aspect of the plug responsible for mechanical strength is called the bridge, while the aspect that controls permeability is called the filter.
Conventional lost circulation materials have typically been made of one or more of the following materials: cellulose, cellulose derivatives, inorganic minerals, synthetic graphite carbon, rubber, thermoplastic polymers, and thermoset polymers. Products containing cellulose, cellulose derivatives, low-temperature melting rubber, and low-temperature melting thermoplastic polymers are generally limited to low-temperature uses. While other materials can be used at higher temperatures, such as synthetic graphite, they tend to be more expensive and to have a lower range of available particle sizes. Because of these limitations, material handling costs tend to be higher for higher-temperature LCMs in order to incorporate a range of material types and particle size categories.
Basalt, a common term used for a variety of volcanic rocks, originates from volcanic magma and flood volcanoes. Basalt forms when a very hot fluid or semifluid material originating under the earth's crust solidifies in open air. Plagiocene and pyroxene make up ˜80% of many types of basalt. Basalt can also contain smaller amounts of silicon dioxide, magnesium oxide, and titanium dioxide; and trace elements such as Zr, Y, Nb, Fe, Ca, K, Na, Sc, Co, La, Ce, Sm, Eu, Yb, Hf, Ta, and Th. Compounds present in basalt may vary depending on the nature and origin of the basalt, especially the SiO2 content. Basalt rocks can be classified by their SiO2 content as alkaline basalts (up to about 42% SiO2), mildly acid basalts (about 42-46% SiO2), and acid basalts (about 46% or greater SiO2). A preferred basalt for continuous, fine, high-strength fiber production is an acid basalt having a SiO2 content of about 46% or greater. Rocks with lower SiO2 content can still be made into fibers. The magma fibers we have used in prototype experiments have shorter lengths, and thus could be made from rock with lower SiO2 content. A preferred basalt for fiber production has an approximately constant composition, the ability to melt without leaving substantial solid residue, an appropriate melt viscosity for fiber formation, and the ability to solidify to a glassy state without marked crystallization. Basalt fibers are typically manufactured by melting the rock, drawing or extruding the melt through a die, cooling, applying lubricant, and winding. Fibers are drawn from the melt under hydrostatic pressure and subsequently cooled to produce hardened filaments. A sizing liquid having components to impart strand integrity, lubrication, and resin compatibility is applied. Filaments are collected together to form a “strand,” and a take-up device winds the filaments onto a forming tube. See generally Jiri Militky, Vladimir Kovacic; ‘Ultimate Mechanical Properties of Basalt Filaments’, Text. Res. J. 66(4), 225-229 (1996); Singha, K. “A Short Review on Basalt Fiber”. International Journal of Textile Science 1(4): 19-28 (2012).
MAGMA™ fiber is a basalt fiber, sold by Lost Circulation Specialists, Inc. (Casper, Wyo.) as an additive for oil well drilling fluids. The major constituents of the Magma™ fiber, as reported by the manufacturer, are CaO 35.7%, MgO 9.6%, Al2O3 9.3% and SiO2 42.3%. The material is thermally stable at temperatures up to about 1,800° F. It has a specific gravity of 2.6, and a solubility of 98.4% in a mixture of 60% hydrochloric acid and 40% acetic acid. MAGMA™ fibers have been used for controlling losses in porous and permeable formations by forming a mat-like bridge over the pore openings. However, small fiber-type LCMs such as MAGMA™ fibers tend not to form permanent bridges within a formation. Pipe movement and fluid movement within a wellbore can readily dislodge bridges over pore openings. MAGMA™ fibers have also been used in conjunction with cross-linked polymers such as polyacrylamides, as well as with water-insoluble polyvinyl alcohol to form mud/polymer/fiber systems for lost circulation control. See Lost Circulation Specialists, Inc. 2010. Magma fiber general information; U.S. Pat. No. 6,581,701; and US patent application publication number: 20100152070.
U.S. Patent Application publication no. 2006/0096759 discloses a lost circulation composition with a first portion of particles having a weight mean particle size of less than about 20 microns, a second portion of particles having a weight mean particle size in the range from about 25 microns to about 400 microns, and a third portion of particles having a weight mean particle size in the range of from about 450 microns to about 1,500 microns.
U.S. Pat. No. 7,229,492 discloses a well cement composition comprising a hydraulic cement, water, and inelastic lost circulation material particles made of granulated waste materials such as polyethylene, polystyrene, or polypropylene.
U.S. Pat. No. 3,574,099 discloses the use of nutshells and asbestos fibers as a lost circulation material.
U.S. Pat. No. 4,579,668 describes a two-component lost circulation material derived from discarded wet-cell battery casings. The first component is a thermoplastic polymer in a flexible, elongated form, and the second component is a granular thermoset plastic with a specific gravity in the range 1.2-1.4.
U.S. Pat. No. 5,826,669 discloses the use of resilient graphitic materials for fluid loss and lubrication purposes.
U.S. Patent Application publication no. 2008/0113879 discloses the use of plastic granules (e.g., polypropylene) as lost circulation additives in drilling fluid.
U.S. Pat. No. 6,581,701 B2 discloses the use of Magma™ fibers in conjunction with cross-linked polymers such as polyacrylamides for lost circulation control.
U.S. Patent Application No. 20100152070 discloses the use of Magma™ Fibers in combination with water-insoluble polyvinyl alcohol in an oil-based mud for lost circulation control.
There is a continuing, unfilled need for improved methods to control lost circulation in wells, especially methods that are adapted for use in high-pressure, high-temperature wells.